feat(convert): support same-format lossless re-encoding

lib/screens/track_metadata_convert.dart

@@ -220,26 +220,14 @@ extension _TrackMetadataConvertAndCueSplit on _TrackMetadataScreenState {
     final currentFormat = _currentFileFormat;
     final isLosslessSource = isLosslessConversionSource(currentFormat);
 
-    final formats = <String>[];
-    if (currentFormat == 'FLAC') {
-      formats.addAll(['ALAC', 'WAV', 'AIFF', 'AAC', 'MP3', 'Opus']);
-    } else if (currentFormat == 'ALAC') {
-      formats.addAll(['FLAC', 'WAV', 'AIFF', 'AAC', 'MP3', 'Opus']);
-    } else if (currentFormat == 'M4A') {
-      formats.addAll(['ALAC', 'FLAC', 'WAV', 'AIFF', 'AAC', 'MP3', 'Opus']);
-    } else if (currentFormat == 'WAV') {
-      formats.addAll(['FLAC', 'ALAC', 'AIFF', 'AAC', 'MP3', 'Opus']);
-    } else if (currentFormat == 'AIFF') {
-      formats.addAll(['FLAC', 'ALAC', 'WAV', 'AAC', 'MP3', 'Opus']);
-    } else if (currentFormat == 'AAC') {
-      formats.addAll(['MP3', 'Opus']);
-    } else if (currentFormat == 'MP3') {
-      formats.addAll(['AAC', 'Opus']);
-    } else if (currentFormat == 'Opus') {
-      formats.addAll(['AAC', 'MP3']);
-    } else {
-      formats.addAll(['AAC', 'MP3', 'Opus']);
-    }
+    final formats = audioConversionTargetFormats
+        .where(
+          (target) => canConvertAudioFormat(
+            sourceFormat: currentFormat,
+            targetFormat: target,
+          ),
+        )
+        .toList(growable: false);
 
     final labels = context.l10n.losslessConversionLabels;
 
@@ -922,9 +910,7 @@ extension _TrackMetadataConvertAndCueSplit on _TrackMetadataScreenState {
             newPath,
           );
           if (convertedMetadata['error'] == null) {
-            convertedBitDepth = readPositiveInt(
-              convertedMetadata['bit_depth'],
-            );
+            convertedBitDepth = readPositiveInt(convertedMetadata['bit_depth']);
             convertedSampleRate = readPositiveInt(
               convertedMetadata['sample_rate'],
             );
@@ -1134,5 +1120,4 @@ extension _TrackMetadataConvertAndCueSplit on _TrackMetadataScreenState {
       }
     }
   }
-
 }

lib/utils/audio_conversion_utils.dart

@@ -128,8 +128,13 @@ bool canConvertAudioFormat({
   required String sourceFormat,
   required String targetFormat,
 }) {
-  if (sourceFormat.trim().toUpperCase() == targetFormat.trim().toUpperCase()) {
-    return false;
+  final normalizedSource = sourceFormat.trim().toUpperCase();
+  final normalizedTarget = targetFormat.trim().toUpperCase();
+  if (normalizedSource == normalizedTarget) {
+    // Same-format lossless re-encoding is useful for reducing bit depth or
+    // sample rate while selecting the dither and resampler implementation.
+    return isLosslessConversionSource(normalizedSource) &&
+        isLosslessConversionTarget(normalizedTarget);
   }
   if (isLosslessConversionTarget(targetFormat) &&
       !isLosslessConversionSource(sourceFormat)) {

test/audio_conversion_utils_test.dart

@@ -0,0 +1,48 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/utils/audio_conversion_utils.dart';
+
+void main() {
+  group('same-format lossless conversion', () {
+    test('allows FLAC re-encoding for quality processing', () {
+      expect(
+        canConvertAudioFormat(sourceFormat: 'FLAC', targetFormat: 'FLAC'),
+        isTrue,
+      );
+      expect(
+        canConvertAudioFormat(sourceFormat: 'flac', targetFormat: ' flac '),
+        isTrue,
+      );
+    });
+
+    test('keeps same-format lossy conversion disabled', () {
+      expect(
+        canConvertAudioFormat(sourceFormat: 'MP3', targetFormat: 'MP3'),
+        isFalse,
+      );
+      expect(
+        canConvertAudioFormat(sourceFormat: 'Opus', targetFormat: 'Opus'),
+        isFalse,
+      );
+    });
+  });
+
+  group('lossless processing options', () {
+    test('preserves both resamplers and both dither methods', () {
+      const soxrTpdf = LosslessConversionProcessing(
+        dither: 'triangular',
+        resampler: 'soxr',
+      );
+      const swrHighPass = LosslessConversionProcessing(
+        dither: 'triangular_hp',
+        resampler: 'swr',
+      );
+
+      expect(soxrTpdf.normalizedResampler, 'soxr');
+      expect(soxrTpdf.normalizedDither, 'triangular');
+      expect(soxrTpdf.hasDither, isTrue);
+      expect(swrHighPass.normalizedResampler, 'swr');
+      expect(swrHighPass.normalizedDither, 'triangular_hp');
+      expect(swrHighPass.hasDither, isTrue);
+    });
+  });
+}